Dataset: Particulate N and NO3 isotopes: EN532

Seawater samples for analysis of the N isotopic composition of nitrate+nitrite and nitrate-only were collected unfiltered at regular depth intervals from the surface to 1000 m in 60 ml (>150 m) or 125 ml (<150 m) square-bottomed, wide-mouth HDPE bottles (Nalgene). Bottles were acid-washed and rinsed with deionized water prior to sampling. At sea, pre-labelled bottles and caps were rinsed three times with sample water, filled to ~85% of the bottle volume, and frozen upright at -20°C until analysis.

Isotopic analyses were conducted using the “denitrifier method”, wherein denitrifying bacteria lacking nitrous oxide (N2O) reductase quantitatively convert nitrate and nitrite in the sample to N2O gas (Sigman et al. 2001, Casciotti et al. 2002) (see also (Weigand et al. in review) for the updated protocol used for analyzing these samples). The isotopic composition of N2O was then measured by gas chromatography-isotope ratio mass spectrometry (GC-IRMS) using a purpose-built on-line N2O extraction and purification system and a Thermo MAT 253 mass spectrometer. Seawater solutions of the international nitrate reference materials, IAEA-N3 and USGS34, as well as an in-house N2O standard, were run in parallel to the samples in order to monitor the quality of bacterial N conversion and mass spectrometric measurements. The reference materials bracketed each group of ~10 samples and were used to correct the measured δ15N to N2 in air (Sigman et al. 2001, Casciotti et al. 2002, McIlvin & Casciotti 2011).       

The measurement of the 15N of nitrate-only for samples with a detectable concentration of nitrite required a nitrite removal pre-treatment. The detection limit for nitrite in this case was 2 nmol kg-1. Samples collected between the surface and ~125 m were treated for nitrite removal via the addition of 10 µl of sulphamic acid solution per ml of sample, which converts sample nitrite to N2 gas with a reaction time of 2-8 minutes, followed by the addition of 5.5 µl of 2M NaOH per ml of sample to restore the pH of the sample to ~7-9 (Granger & Sigman 2009). The pooled standard error for 15N was 0.04‰ and 0.11‰ (n ≥3) for nitrate+nitrite and nitrate concentrations ≥0.5 µmol l-1 and <0.5 µmol l-1, respectively. Hereafter, “nitrate” in the text refers to nitrate-only, after the subtraction (for concentration) or removal (for 15N) of nitrite.

Suspended particulate N: Suspended PN was collected at various depths throughout the euphotic zone, including within the surface mixed layer and at the depth of maximum chlorophyll concentration, by gentle vacuum filtration (<135 mbar), of 8 l of seawater through a GF-75 filter. Filters were transferred to pre-combusted (500°C for 5 h) aluminium foil envelopes, and immediately frozen at -80°C until analysis. In the laboratory, the PN filters were dried in a desiccating oven at 40°C. Three subsamples were cored from each filter and transferred to combusted 4 mL glass Wheaton vials. PN was oxidised to nitrate using the persulphate oxidation method of Knapp et al. (2005), and as modified by Fawcett et al. (2011; 2014); this was conducted in a laminar flow hood equipped with an ammonia/amine filter. Briefly, 2 ml of persulphate oxidizing reagent (POR) were added to each sample vial, as well as to triplicate vials containing a filter blank plus varying quantities of two L-glutamic acid isotope standards, USGS-40 and USGS-41 (Qi et al. 2003); this allows determination of the N content and 15N of the POR+filter blank. The POR was made by dissolving 2.5 g of 4× recrystallised, methanol-rinsed potassium persulphate and 2.5 g of sodium hydroxide in 100 ml of ultra high-purity deionised water. Following POR addition, vials were autoclaved at 121°C for 55 minutes on a slow-vent setting, after which sample pH was lowered to 5-8 using 12N HCl. The concentration and δ15N of the resultant nitrate was measured via chemiluminescent analysis (Braman & Hendrix 1989) and the denitrifier method (see above) (Sigman et al. 2001, Casciotti et al. 2002). The final N content and δ15N of the oxidised samples was corrected for the POR+filter blank. N content was converted to PN concentration by normalising to whole-filter area and volume of seawater filtered.

References:

Braman RS, Hendrix SA (1989) Nanogram nitrite and nitrate determination in environmental and biological materials by vanadium(iii) reduction with chemi-luminescence detection. Anal Chem 61:2715-2718

Casciotti K, Sigman D, Hastings MG, Böhlke J, Hilkert A (2002) Measurement of the oxygen isotopic composition of nitrate in seawater and freshwater using the denitrifier method. Anal Chem 74:4905-4912

Fawcett SE, Lomas M, Casey JR, Ward BB, Sigman DM (2011) Assimilation of upwelled nitrate by small eukaryotes in the Sargasso Sea. Nature Geoscience 4:717-722

Fawcett SE, Lomas MW, Ward BB, Sigman DM (2014) The counterintuitive effect of summer‐to‐fall mixed layer deepening on eukaryotic new production in the Sargasso Sea. Glob Biogeochem Cycle 28:86-102

Granger J, Sigman DM (2009) Removal of nitrite with sulfamic acid for nitrate N and O isotope analysis with the denitrifier method. Rapid Commun Mass Spectrom 23:3753-3762

Knapp AN, Sigman DM, Lipschultz F (2005) N isotopic composition of dissolved organic nitrogen and nitrate at the Bermuda Atlantic Time‐series Study site. Glob Biogeochem Cycle 19

McIlvin MR, Casciotti KL (2011) Technical updates to the bacterial method for nitrate isotopic analyses. Anal Chem 83:1850-1856

Qi H, Coplen TB, Geilmann H, Brand WA, Böhlke J (2003) Two new organic reference materials for δ13C and δ15N measurements and a new value for the δ13C of NBS 22 oil. Rapid Commun Mass Spectrom 17:2483-2487

Sigman D, Casciotti K, Andreani M, Barford C, Galanter M, Böhlke J (2001) A bacterial method for the nitrogen isotopic analysis of nitrate in seawater and freshwater. Anal Chem 73:4145-4153

Weigand MA, Foriel J, Barnett B, Oleynik S, Sigman DM (in review) Updates to instrumentation and protocols for isotopic analysis of nitrate by the denitrifier method. Rapid Commun Mass Spectrom